Heterodyne refractive index sensor using hybrid external cavity lasers

Abstract

Disclosed is a refractive index sensor that includes a reference laser and a RI sensitive laser wherein the reference and RI sensitive lasers have reference and sensing oscillating frequencies respectively. The reference laser is configured to maintain the oscillating frequency at a constant value, upon exposure of the refractive index senor to an input sample, and the RI sensitive laser has the sensing oscillating frequency that varies based on a refractive index of the input sample, upon exposure to the input sample. The reference laser and RI sensitive lasers generate reference and sensing signals, and a photodiode receives and combines the reference and sensing signals to generate a combined signal that has a beat frequency proportional to the refractive index of the input sample. A computing unit determines the refractive index of the input sample based on the beat frequency.

Description

[0001] Title

[0002] Heterodyne refractive index sensor using hybrid external cavity lasers

[0003] Field

[0004] The present disclosure relates to a refractive index sensor, and more specifically to a heterodyne refractive index sensor using hybrid external cavity semiconductor lasers.

[0005] Refractometers are used in a wide range of industries. Refractometers play a role in routine sample analysis of fresh produce, dietary food and beverage products, alcoholic beverages, and sugar concentrates. There is a growing demand for refractometers in healthcare. For example, monitoring individual protein levels of serum and plasma is emerging as a prominent application. Water quality testing is currently carried out offline involving the collection of samples that are sent away for analysis and often takes 2- 3 days to complete, delaying the identification and resolution of faults. Further, air quality analysis is an important market, and many high-performance sensors use quantum cascade laser (QCL) based systems- a highly specialised laser with a high unit cost. Refractive index sensors are also key components in many types of photo-thermal spectroscopy system.

[0006] The monitoring a substance’s refractive index is a powerful method of optical sensing as many contaminants and processes affect the refractive index. Monitoring the RI index of a substance can also provide information on its temperature, pressure and other physical parameters which can be of critical importance in environmental monitoring, in sensing, or in processes in chemical / manufacturing plants. The response of an optical resonator is linked to the refractive index of the materials of which it is composed. In an example, there is a technology based on silicon ring resonators, which detects the small change in resonant wavelength that is induced when material is deposited on the ring. The surface of the ring is functionalised with a range of antigens that are specific to different target compounds. When patient samples are run over the chip, the macromolecules bond to the surface changing the resonance wavelength. Further, Mach Zehnder interferometer based refractive index (RI) measurements are used to monitor the dynamics of water quality in drinking water distribution networks. The RI is a generic indicator of water quality as any substance that is dissolved in water will change the water’s refractive index. Thus, monitoring the RI allows all chemical changes to be tracked regardless of the type, unlike other sensors, each of which are sensitive to only a small subset of possible contaminants. The RI sensing can therefore provide event monitoring giving early warning of problems in the network.

[0007] The detection limit (DL) is the key measure of the performance of a RI sensor and determines the smallest change in refractive index that a sensor can detect. The DL is a function of the RI sensitivity, which is the magnitude in shift of the resonant wavelength versus the change in RI of the sample, and of the sensor resolution, which characterizes the smallest possible spectral shift that can be accurately measured. FIG.l illustrates the origin of the detection limit, which is proportional to the Full width Half maximum (FWHM) of the spectral feature. The sensitivity depends on the fraction of the optical mode that interacts with the sample. The sensor resolution depends on precision with which an extrema in the transmission spectrum can be tracked, which will be proportional to Q-factor of the resonator. The detection limit is commonly defined as DL=R / S, where R is the sensor resolution i.e. the minimum spectral shift that can be resolved and S is the is the sensitivity, the shift in resonance wavelength with change in refractive index. The resolution R is determined the standard deviation of the spectral variations, which is proportional to the FWHM of the resonance and inversely proportional to signal to noise ratio [I. M. White and X. Fan, “On the performance quantification of resonant refractive index sensors,” Optics Express 16(2), 1020-1028 (2008).]. R is a function of the entire system performance though for simplicity and to allow the use of the detection limit as a figure of merit, R is often taken to be the FWHM. Based on this criteria, typical values for the detection limit are on the order of IxlO'6Refractive index unit (RIU) for silicon micro-ring based sensors, which corresponds to concentrations in the parts per million (ppm) range. While these are powerful techniques, parts per billion sensitivities are required to detect many trace substances and contaminants.

[0008] WO 2018 / 104938 describes a sensing method and a sensing system and an RF source thereof comprising two frequency locked lasers coupled to two resonance cavities, one being configured as a sensing element being sensitive to environmental perturbations to be measured, and the second being a reference resonator. However, this method is based on wavelength-modulated source interacting with the sensing cavity and is quite complex, noisier, less scalable and less sensitive.

[0009] CN 103 018200 relates to a monitoring device and a monitoring method for the refractive index of a transparent medium and relies on the assembly of discrete (bulk) optical components, which limits the potential for scalable production and has limited robustness. WO 01 / 98756 relates to an integrated optical chip device for molecular diagnostics comprising a tunable laser cavity sensor chip using heterodyned detection at the juncture of a sensor laser and a reference laser, the sensor laser including exposed evanescent field material. XP 040192764 titled as “Biophotonic Integrated circuits” disclose biophotonic integrated circuits built on both GaAs and InP substrates, incorporating widely tunable lasers, novel evanescent field sensing waveguides, heterodyne spectrometers, and waveguide photodetectors, suitable for high sensitivity transduction of affinity assays. These documents consider traditional lasers, such as distributed feedback lasers, which even with modification have an inherently low sensitivity to environmental refractive index changes.

[0010] XP 004151882 titled as “Design and characteristics of DBR-laser-based environmental sensors” discloses an optical sensor that is based on the modification of the resonant cavity of a distributed-Bragg-reflector (DBR) laser. XP093217387 titled as “High-Q asymmetrically cladded silicon nitride ID photonic crystals cavities and hybrid external cavity lasers for sensing in air and liquids” discloses a high Q-factor silicon nitride (SiN) ID photonic crystal (PhC) cavities side-coupled to curved waveguides, operating with both silica and air cladding. In those works, the changes in wavelength are monitored using a spectrometer which has an inherently low spectral resolution leading to poor detection limits or interferometer which are large and cumbersome

[0011] In an aspect of the present invention, there is provided a laser, the emission wavelength of which depends on the refractive index of the surrounding material, and a means to detect very small changes in the emission wavelength of the said laser, by mixing the output of the refractive index sensing laser with that of a reference laser. The mixing of the two outputs produces a beat frequency, with a value that is given by the difference between the two frequencies, which is monitored by a suitable photodiode (also known as heterodyne detection in the literature). A change in the cladding material changes the frequency of the RI sensitive laser creating a change in the beat frequency.

[0012] According to the invention there is provided, as set out in the appended claims, a refractive index sensor that includes a reference laser and a RI sensitive laser wherein the reference and RI sensitive lasers have reference and sensing oscillating frequencies respectively, wherein the reference laser is configured to maintain the oscillating frequency at a constant value, upon exposure of the refractive index senor to an input sample, wherein the RI sensitive laser has the sensing oscillating frequency that varies based on a refractive index of the input sample, upon exposure to the input sample, wherein the reference laser and RI sensitive lasers generate reference and sensing signals. The refractive index sensor further includes a photodiode for receiving and combining the reference and sensing signals to generate a combined signal that has a beat frequency proportional to the refractive index of the input sample, and a computing unit for determining the refractive index of the input sample based on the beat frequency.

[0013] In an embodiment of the present invention, the RI sensitive laser includes a hybrid external cavity laser formed of a gain chip with a refractive index sensitive element that controls the operating wavelength of the RI sensitive laser.

[0014] In an embodiment of the present invention, the RI sensitive laser includes a hybrid external cavity laser formed of a one-dimensional photonic cavity and a gain chip, wherein the one-dimensional photonic cavity acts as a resonant mirror that sets the operating wavelength of the RI sensitive laser.

[0015] In an embodiment of the present invention, the reference laser includes a hybrid external cavity laser formed of a gain chip with a refractive index sensitive element that controls the operating wavelength of the RI sensitive laser, and in that the refractive index sensitive element has a thick silicon oxide cladding to ensure no changes to the reference oscillating frequency upon exposure to the input sample. In an embodiment of the present invention, each of the RI sensitive and reference lasers have linewidth of order of MHz or less.

[0016] In an embodiment of the present invention, the one-dimensional photonic crystal cavity is silicon-nitride based.

[0017] In an embodiment of the present invention, each of the RI sensitive and reference lasers has lasing wavelengths, such that the difference in respective frequencies is less than the bandwidth of the photodiode.

[0018] In an embodiment of the present invention, the RI sensor is situated within a gas cell.

[0019] In an embodiment of the present invention, the photodiode is integrated either monolithically, using heterogeneous integration, hybrid integration or using flip chip bonding on the chip.

[0020] In an embodiment of the present invention, the RI sensitive laser has the sensing oscillating frequency that varies based on a refractive index change generated by temperature, through the thermo-optic mechanism, so that the RI sensor may be used for sensitive temperature measurements

[0021] In an embodiment of the present invention, each of the RI sensitive and reference lasers are configured to be exposed to similar fluctuations in conditions external to the refractive index sensor.

[0022] In an embodiment of the present invention, the RI sensitive laser includes a Distributed Feedback (DFB) laser phase- stabilized, self-injection-locked DFB orFabry-Perot (FP) lasers.

[0023] In an embodiment of the present invention, each of the RI sensitive and reference lasers have arbitrarily narrow linewidths down to 1 Hz.

[0024] In an embodiment of the present invention, an additional layer of material with thermooptic co-efficient different to that the waveguide material is deposited on the reflector or waveguide of the reference laser, to enable appropriate matching of the wavelengths of the reference and sensing lasers. By varying the system temperature, the oscillating frequencies of the reference and sensing lasers tune at different rates, and thus their difference can be adjusted. The additional layer could be a polymer layer or a glass coating.

[0025] In another aspect of the present invention, there is provided a method of estimating a refractive index of an input sample. The method includes providing a reference laser and a RI sensitive laser; generating a reference signal by the reference laser upon exposure of the reference laser to the input sample, wherein the reference signal has a constant reference frequency; generating a sensing signal by the RI sensitive laser upon exposure of the RI sensitive laser to the input sample, wherein the sensing signal has a sensing oscillating frequency that varies based on a refractive index of the input sample; combining the reference and sensing signals by a photodiode, to generate a combined signal that has a beat frequency proportional to the refractive index of the input sample; and estimating the refractive index of the input sample based on the beat frequency.

[0026] In yet another aspect of the present invention, there is provided a refractive index sensor that includes a reference laser and a RI sensitive laser wherein each of the reference and RI sensitive lasers is a hybrid external cavity laser formed of a gain element, a waveguide and a reflector, and wherein the reference laser and RI sensitive laser are realised on a single chip, for exposure to identical ambient conditions The reflector of the RI sensitive laser includes an RI sensitive element for varying the oscillating frequency of the RI sensitive laser based on the refractive index of an input analyte, upon exposure of the RI sensitive laser to the input analyte. The reference laser is configured to maintain corresponding oscillating frequency at a constant value, upon exposure to the input analyte, and wherein the waveguide and reflector of the reference laser are covered with a cladding layer to prevent interaction of the reference laser with the input analyte and enable the reference laser to maintain constant oscillating frequency, wherein the reference laser and RI sensitive lasers generate reference and sensing signals. The refractive index sensor further includes a photodiode for receiving and combining the reference and sensing signals to generate a combined signal that has a beat frequency proportional to the refractive index of the input analyte, and a computing unit for determining the refractive index of the input sample based on the beat frequency. In an embodiment of the present invention, the cladding layer includes a thick silicon oxide cladding to maintain constant corresponding oscillating frequency upon exposure to the input analyte.

[0027] In an embodiment of the present invention, the hybrid external cavity laser includes a photonic crystal, micro-ring resonator, distributed bragg reflector or similar photonic integrated circuit component.

[0028] In an embodiment of the present invention, an additional layer of different thermo-optic co-efficient is deposited on the reflector or waveguide of the reference laser, to enable appropriate matching of the wavelengths of the reference and sensing lasers by varying system temperature, and wherein the additional layer is a polymer layer or a glass coating.

[0029] In an embodiment of the present invention, the RI sensitive laser has the oscillating frequency that varies based on at least one of: a refractive index change measured directly in the material of the analyte, or indirectly via the temperature change inducted in the material of the RI sensitive laser, a refractive index change generated by pressure of the fluid interacting with the RI sensitive laser, a refractive index change generated by concentration of the fluid interacting with the RI sensitive laser, or a refractive index change induced by the photothermal effect.

[0030] In an embodiment of the present invention, the sensing laser and reference laser are narrow linewidth lasers self-injection locked to Photonic integrated circuit based resonator.

[0031] In an embodiment of the present invention, the refractive index sensor comprises a signal generator for generating a reference signal of a given reference frequency, and an external source to cause further time- varying RI change either directly or indirectly in the input analyte based on the reference frequency, and wherein the photodiode and computing unit performs phase- sensitive detection of the beat signal between the RI sensitive laser and the reference laser by locking onto the known reference frequency, to generate the amplitude and phase of the refractive index change with high sensitivity, in presence of significant noise. In an embodiment of the present invention, the evanescent tail of a mode, or portion of the mode, of the RI sensitive laser interacts with the input analyte to cause changes in the oscillating frequency.

[0032] In yet another aspect of the present invention, there is provided a method of estimating a refractive index of an input analyte. The method includes providing a reference laser and a RI sensitive laser, wherein each of the reference and RI sensitive lasers is a hybrid external cavity laser formed of a gain element, a waveguide and a reflector, and wherein the reference laser and RI sensitive laser are realised on a single chip, for exposure to identical ambient conditions; covering the waveguide and reflector of the reference laser with a cladding layer to prevent interaction of the reference laser with the input analyte and enable the reference laser to maintain constant oscillating frequency upon exposure to the input analyte; varying an oscillating frequency of the RI sensitive laser based on the refractive index of the input analyte, upon exposure to the input analyte, wherein the reflector of the RI sensitive laser includes an RI sensitive element; generating reference and sensing signals by reference and laser upon exposure of the reference laser to the input analyte, wherein the reference signal has a constant reference frequency, and the sensing signal has a sensing oscillating frequency that varies based on the refractive index of the input sample; combining the reference and sensing signals by a photodiode, to generate a combined signal that has a beat frequency proportional to the refractive index of the input analyte; and estimating the refractive index of the input sample based on the beat frequency.

[0033] Various embodiments of the present invention provide a refractive index (RI) sensor that has a refractive index detection limit improved by 3 orders of magnitude relative to the state of the art. In one embodiment, the RI sensor implements an optical heterodyne measurement set-up to monitor a hybrid external cavity semiconductor laser. The hybrid external cavity laser uses a resonant mirror comprised of a one-dimensional (ID) photonic crystal cavity (PhC), a waveguide and a gain chip (also known as a reflective semiconductor optical amplifier). A significant portion mode supported by the ID PhC overlaps with the upper cladding material (in this case, the gas or liquid of the sample), which means that the lasing wavelength is strongly affected by the sample’s refractive index. The linewidth of the laser is of the order of megahertz which is at least three orders of magnitude smaller than that of silicon ring resonators typically used for RI sensing. The output of this laser is combined with that of a reference laser to create a beat signal to read out the changes in lasing wavelength caused by changes in the ambient RI. Lasers with linewidths in kilohertz range and smaller have been demonstrated and thus very low detection limits are possible. The beat frequency depends on the difference in wavelength between the two lasers and will typically be in 0.1-50GHz range. The beat frequency is monitored using a photodiode with a suitable frequency response. Further, the two lasers are frequency-locked and thermally and electrically stabilized to generate the beat frequency. The use of a hybrid external cavity laser with a photonic crystal resonator engineered for strong optical mode overlap with the sensing environment, yield orders of magnitude higher sensitivity. Also, the combination of heterodyne detection and RI sensitive lasers is inventive as it introduces a non-traditional sensing principle, a novel laser configuration, and achieves significant performance gains.

[0034] In a preferred embodiment, the RI sensing laser, the reference laser and the photo-diode are realised on a single silicon chip, so that both the lasers are exposed to identical ambient conditions. The lasers may be connected using waveguides using photonic integration techniques, that may include micro-transfer printing. Other integration techniques may be employed such as monolithic or heterogeneous or hybrid integration approaches (Template Assisted Selective Epitaxy, re-growth) or using die-bonding, wafer-bonding and flip-chip bonding on the chip.

[0035] The waveguides may be comprised of silicon or silicon nitride amongst others.

[0036] It should be clear to some knowledgeable in the art, that heterodyne measurement can be applied to any laser type the wavelength of which is sensitive to the cladding material.

[0037] The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which: FIG.1 illustrates an origin of the detection limit, which is proportional to the Full width Half maximum (FWHM) of the spectral feature of an optical device;

[0038] FIG.2 is a block diagram illustration of an RI sensor, in accordance with an embodiment of the present invention;

[0039] FIG.3 is a three-dimensional view of a one-dimensional photonic cavity laser for use in the RI sensor of FIG.2, in accordance with an embodiment of the present invention;

[0040] FIG.4 illustrates a refractive index sensor formed using a pair of photonic cavity lasers, based on an on-chip optical heterodyne measurement technique, in accordance with an embodiment of the present invention;

[0041] FIG. 5 provides an example measurement showing the refractive index of an input sample proportional to the beat frequency; and

[0042] FIG.6 illustrates an embodiment of the RI sensor, in which an additional layer of material is deposited on a reflector / resonant mirror of the reference laser, in accordance with an embodiment of the present invention.

[0043] Detailed Description of the Drawings

[0044] FIG.2 illustrates a refractive index sensor 200 based on an optical heterodyne measurement technique, in accordance with an embodiment of the present invention.

[0045] The refractive index sensor 200 includes an RI sensitive laser 202, a reference laser 204, and a photodiode 206. A computing unit 208 may be provided external to the refractive index sensor 200 or integrated within it. The RI sensitive laser 202 and reference laser 204 have reference and sensing oscillating frequencies respectively.

[0046] The reference laser 204 is constructed so that it experiences no changes to the reference oscillating frequency upon exposure to the input sample. Examples of the input sample include gases that flow across sensor chip, for example methane in standard air, pesticides in water etc. The input analyte sample may be in liquid, solid or gas phase. The sample may be introduced via a variety of methods, that include pumping the sample gas or fluid through a flow cell that contains the RI sensor 200, or by pumping the sample through channels or pipes that are attached to the RI sensors or in which the RI sensor 200 is embedded. The RI sensitive laser 202 has the sensing oscillating frequency that changes based on a refractive index of the input sample, wherein the reference laser 204 and RI sensitive laser 202 generate reference and sensing signals respectively. In an embodiment of the present invention, the RI sensitive laser 202 has the sensing oscillating frequency that varies based on at least one of: a refractive index change measured directly in the material of the analyte, or indirectly via the temperature change inducted in the material of the RI sensitive laser, a refractive index change generated by pressure of the fluid interacting with the RI sensitive laser, a refractive index change generated by humidity of the fluid interacting with the RI sensitive laser, and a refractive index change generated by concentration of the fluid interacting with the RI sensitive laser. Further, each of the RI sensitive and reference lasers are constructed so that they are exposed to same fluctuations in the conditions external to the refractive index sensor. Thus, the transient changes in RI triggered by an external event are measured.

[0047] In an embodiment of the present invention, each of the RI sensitive and reference lasers have arbitrarily narrow linewidths down to the 1 Hz, such as the one exhibited by selfinjection locked DFB lasers and phase-controlled spiral lasers. In an example, the RI sensitive laser and reference lasers are narrow linewidth self-injection lasers locked to chip based resonator.

[0048] In an embodiment of the present invention, one or both reference and RI sensitive lasers may be integrated either monolithically or using heterogeneous or hybrid integration approaches. Examples of such approaches include, but are not limited to, ( / / Transfer- Printing, Template Assisted Selective Epitaxy, re-growth) or using die-bonding, waferbonding and flip-chip bonding on the chip.

[0049] The photodiode 206 receives and combines the reference and sensing signals to generate a combined signal that has a beat frequency proportional to the refractive index of the input sample. The computing unit 208 determines the refractive index of the input sample based on the beat frequency. In an embodiment of the present invention, each of the RI sensitive laser 202 and the reference laser 204 have nominally identical lasing wavelengths, which mean the difference of their frequencies should be less than the bandwidth of the photodiode 206. In an example, if the bandwidth of the photodiode is 35GHz, the wavelengths of both lasers should differ by a fraction of this e.g. 30GHz.

[0050] Examples of the RI sensitive laser 202 include but are not limited to: a Distributed Feedback (DFB) laser, including phase-stabilized and self-injection-locked DFB and FP lasers.

[0051] Further, the RI sensitive laser 202 may include a hybrid external cavity laser formed of a gain chip and a) a Distributed Bragg Grating (DBR), 7T-shifted DBR or sampled DBR, b) a racetrack or a microring resonator (MRRs), or a series of MRRs and racetrack resonators, included configurations such as Sagnac loops, Vernier loops and add / drop routing, c) an injection-locked Fabry-Perot (FP) cavity, d) a grating integrated MRR or racetrack resonator and e) a combination of a MRR or racetrack resonator and a grating (such as a DBR), wherein a), b), c), d) and e) act as a resonant mirror that sets the operating wavelength of the RI sensitive laser 202.

[0052] Also, both the RI sensitive laser 202 and the reference laser 204 may use a hybrid external cavity laser which is described in detail below.

[0053] Hybrid External cavity lasers offer advantages over other types of semiconductor laser, as the monolithic distributed feedback lasers, as the hybrid nature enables independent optimization and fabrication of the active and passive regions while allowing cost- effective use of III-V gain materials. This alleviates problems commonly encountered in wafer-bonded or heterogeneously integrated lasers, which arise from the difficulties in simultaneously realizing evanescent coupling, good thermal conductivity, and high gain. Further to that, both the silicon and the gain chips can be tested and screened prior to assembly to improve yields. Photonic crystals (PhC) lasers are another variant of hybrid external cavity lasers. The hybrid external cavity laser may include a photonic crystal, micro-ring resonator, distributed bragg reflector, or other equivalent photonic integrated circuit component. In an embodiment of the present invention, the RI sensor 200 further includes a signal generator 201, and an external source 203 such as an ultrasound generator, pressure wave generator, or sound wave generator. The external source 203 may be a laser of a given oscillating frequency and modulated at a known frequency, and may create an additional RI modulation in the analyte by means of the photothermal effect, etc. Where the RI change in the analyte is induced by an external modulated source 203, the beat signal between the sensing laser and the reference laser may be analysed using synchronous detection techniques. The signal generator 201 that drives the modulation provides a known reference frequency. This reference can be used in a lock-in amplifier or equivalent FPGA -based signal processing system to perform phase-sensitive detection of the beat frequency. The effect of the external modulated source 203 on the analyte causes a time-varying refractive index, either directly or indirectly via for example, pressure, temperature which leads to a modulation of the lasing wavelength of the RI sensitive laser. When this modulated sensor output is mixed with the reference laser output, it produces a beat frequency that carries the signal. By locking onto the known modulation frequency, one can extract the amplitude and phase of the refractive index change with high sensitivity, even in the presence of significant noise, reducing in particular the flicker (1 / f) noise.

[0054] FIG.3 is a three-dimensional view of a typical hybrid external cavity laser 300 that may be used in an embodiment of the RI sensor 200, in accordance with an embodiment of the present invention. An example of the hybrid external cavity laser 300 is a PhC laser. The hybrid external cavity laser 300 includes a III-V reflective semiconductor amplifier also known as a gain chip 302 and a silicon nitride based one-dimensional photonic crystal (PhC) 304 which acts as a resonant mirror that sets the operating wavelength. The hybrid external cavity laser 300 achieves side mode suppression ratio greater than 40 dB, with threshold currents<20 mA and wall plug efficiencies of almost 10%. It is to be noted that approximately 40% of the mode supported by the ID PhC 304 exists in the top cladding material (in this case, the air or water of the sample) meaning the lasing wavelength is affected by the sample’s refractive index giving a sensitivity of approx. 140nm / RIU. The linewidth of the hybrid external cavity laser 300 is narrow, approximately 4xlOA6MHz, which is about 3 orders of magnitude narrower than the silicon-based ring resonators in use, enabling extremely precise RI measurements. FIG.4 illustrates a refractive index sensor 400 based on an on-chip optical heterodyne measurement technique, in accordance with another embodiment of the present invention.

[0055] The refractive index sensor 400 includes first and second hybrid external cavity lasers 402 and 404 fabricated on a silicon chip, wherein each hybrid external cavity is similar to the hybrid external cavity 300. An example of hybrid external cavity laser is a PhC laser. Further, each of the first and second PhC lasers 402 and 404 have nominally identical lasing wavelengths, with frequency difference less than the bandwidth of the employed photodiode 406. In an embodiment of the present invention, the photo-diode (PD) 406 is integrated either monolithically (via a silicon germanium photodiode) or using flip chip bonding of a III-V diode. The photo-diode (PD) 406 may be integrated either monolithically (via a silicon germanium photodiode) or using flip chip bonding of a III- V diode. The gain chip (Reflective Semiconductor Optical Amplifiers) may be flip chip bonded to the chip.

[0056] The first hybrid external cavity laser 402 acts as a local oscillator with first oscillating frequency fi and has a thick silicon dioxide cladding. The first hybrid external cavity laser 402 has a thick silicon dioxide cladding over its resonant mirror which makes it insensitive to the refractive index of an input sample. Due to thick silicon dioxide cladding, the evanescent wave of the supported mode(s) do not interact with the analyte, so that the oscillating frequency remains at a constant value, upon exposure to the input analyte.

[0057] The second hybrid external cavity laser 404 does not have a silicon dioxide cladding over its resonant mirror and thus interacts with the input sample. The input sample may be brought into contact with the silicon chip using many different methods that may include suitable flow channels. The second hybrid external cavity laser 404 has a second oscillating frequency fi that depends on the RI of the input sample.

[0058] The first and second hybrid external cavity lasers 402 and 404 generate first and second light signals upon exposure to the input sample. The first and second light signals are combined on-chip producing a beat frequency on a high-speed photo-diode 406 in the gigahertz range. The combined signal may be analysed using a computing device such as data acquisition device, and balanced detection (not shown) may be implemented to reduce noise. The beat frequency fbeatmaY have a value :

[0059] In all embodiments, the change in the RI of the input sample changes the oscillating frequency fi according to:

[0060] Where FWHM is the full width at half maximum of the resonance of the resonant mirror, FSR is the free spectral range between the modes of the laser cavity, f2 is the original oscillating frequency of the PhC laser 404, An is the change in effective refractive index, and n is the effective refractive index.

[0061] Thus, the change in beat frequency is thus proportional to change in change in effective refractive index:

[0062] The resolution of the measurement is given by the linewidth of the hybrid external cavity lasers. It would be apparent to one of ordinary skill in the art, that while the line width of the hybrid external cavity lasers 402 and 404 is approx. 1 MHz, this is no way limiting.

[0063] In an example, for a linewidth of 1MHz, a detection limit of 2.5xl0'9RIU is estimated. FIG 5 illustrates an example measurement, in which the refractive index is varied through variations in a gas mixture of nitrogen and carbon dioxide, showing the beat frequency as a function of the cladding refractive index. In the case, the cladding is a gas comprising a controllable mixture of nitrogen and carbon dioxide. The gas refractive index is controlled by varying the composition of the gas from n= 1.00029442 for pure nitrogen and n= 1.00043822 for pure carbon dioxide.

[0064] It is to be noted that the changes of RI can be generated, and therefore accurately measured by the heterodyne sensor, by either the concentration of the fluid interacting with the RI sensor, or its temperature, pressure and humidity, according to the relation: Where TOC is the thermo-optic coefficient of the material on which the RI sensitive laser is based on, k is constant related to the fluid sensed and Ap is the change in pressure of the fluid.

[0065] It is well known that the temperature induced spectral fluctuations limit the performance of RI sensors based on high Q-factor resonators, and considerable effort must be expended to stabilise the operating temperature. In this respect, the heterodyne measurement has an important advantage in that the beat frequency is a function of only the difference in output wavelengths of the RI sensing laser and reference laser. Fluctuations in the environmental conditions that affect both lasers thus have no effect on the measurement. This feature can be considered as a form of balanced detection, and a means to eliminate common mode noises.

[0066] In an embodiment of the present invention, two on-chip lasers with careful temperature and current control operate with a stable beat frequency. By integrating both lasers side by side, the lasers are exposed to very similar environments and temperatures and thus the effects of external temperature fluctuations. The RI sensor 400 may use a common low noise battery source that may drive both lasers. The effects of current variations may be almost completely cancelled out. At least an order of magnitude improvement in stability relative for free -running operation may be achieved. The ambient temperature fluctuations are a major problem for all refractometers, and therefore needs to be compensated.

[0067] The RI sensor 400 provides provide the means to measure changes in refractive index with extremely high precision. The RI sensor 400 may realise one of two potential demonstrators: a single gas sensor chip capable of detecting methane (or similar gas) at sub lOppm levels in real-time; and a water quality sensor that can detect trihalomethanes, pesticides in water at sub ppm levels. The gas sensor may be a self-contained unit with a footprint of 5x5cm. The RI sensor 400 has a superior detection limit which may allow the detection of water contaminants in the parts per billion range, expanding the range of compounds that may be detected to include trihalomethanes, pesticides and others. Such compounds are virtually impossible to detect on-line in real-time with the current technology and a network of the refractive index sensors of the present invention may provide unprecedented surveillance of a water network. Also, the refractive index sensor is configured to detect a broad spectrum of gases at concentrations of lOppm or better.

[0068] The RI sensor 400 may find applications in numerous market verticals such as Air Quality, Water Quality, Blood Diagnostics. The lower detection limit enables detection of other compounds allowing the potential detection of pesticides, one of the most challenging contaminants to detect, giving a competitive advantage. Laser stability may determine the detection limit in practice, which is estimated to be ~lxl0'8RIU in the worst case- representing a lOOx improvement relative to silicon ring resonators.

[0069] The RI sensor may replace the RI transducer used in other systems, such that used in interferometric assisted photothermal spectroscopy which typically uses a Fabry-Perot (FP) interferometer as a sensitive transducer for detection of induced refractive index changes in the gas sample. The excitation laser beam is tuned to an absorption line of the target analyte and modulated. The periodic excitation of the analyte molecules inside the FP cavity through absorption of laser radiation leads to a periodic modulation of the gas temperature due to photothermal heating. A change of the gas temperature in turn causes a change of the refractive index. The periodic transmission function of the interferometer is shifted with respect to the vacuum wavelength when the refractive index of the sample in between the two mirrors alters. The ICAPS technique is relatively new, and can be applied to a very wide range of gases. High sensitivities have been achieved e.g. 5ppb levels for SO2 and 15ppb for CO. ICAPS is a generic technology as it can be used for many gases including methane, ethane and similar molecules. The use of RI sensor disclosed here potentially gives orders of magnitude improvements in sensitivity.

[0070] In practice, it is often difficult to fabricate lasers with the required wavelength separation (i.e. within the bandwidth of photodetector). The use of microheaters to tune the laser wavelengths is possible but adds complexity to the system, introduces noise, and is not applicable in certain sensing configurations.

[0071] FIG.6 illustrates an embodiment of the RI sensor 600, in which an additional layer of material is deposited on a reflector / resonant mirror of the reference laser, in accordance with an embodiment of the present invention. The additional layer may include a polymer layer, or a glass coating. The additional layer has a thermo-optic coefficient different from the material of the reflector. Generally, the chip temperature is controlled via a heating or cooling element, and when the chip temperature is changed, the wavelengths of the reference and sensing laser may change at different rates and thus they may be brought the required wavelength separations. In an embodiment of the present invention, the additional layer of material with thermo-optic co-efficient different to that the waveguide material is deposited on the reflector or waveguide of the reference laser, to enable appropriate matching of the wavelengths of the reference and sensing lasers. By varying the system temperature, the oscillating frequencies of the reference and sensing lasers tune at different rates, and thus their difference can be adjusted. The additional layer could be a polymer layer or a glass coating.

[0072] In the specification the terms "comprise, comprises, comprised and comprising" or any variation thereof and the terms include, includes, included and including" or any variation thereof are considered to be totally interchangeable, and they should all be afforded the widest possible interpretation and vice versa.

[0073] The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.

Claims

Claims:

1. A refractive index sensor, comprising: a reference laser and a RI sensitive laser wherein each of the reference and RI sensitive lasers is a hybrid external cavity laser formed of a gain element, a waveguide and a reflector, and wherein the reference laser and RI sensitive laser are realised on a single chip, for exposure to identical ambient conditions, wherein the reflector of the RI sensitive laser includes an RI sensitive element for varying the oscillating frequency of the RI sensitive laser based on the refractive index of an input analyte, upon exposure of the RI sensitive laser to the input analyte, wherein the reference laser is configured to maintain corresponding oscillating frequency at a constant value, upon exposure to the input analyte, and wherein the waveguide and reflector of the reference laser are covered with a cladding layer to prevent interaction of the reference laser with the input analyte and enable the reference laser to maintain constant oscillating frequency, and wherein the reference laser and RI sensitive lasers generate reference and sensing signals; a photodiode for receiving and combining the reference and sensing signals to generate a combined signal that has a beat frequency proportional to the refractive index of the input analyte; and a computing unit for determining the refractive index of the input sample based on the beat frequency.

2. The refractive index sensor as claimed in claim 1, wherein the hybrid external cavity laser is formed of a one-dimensional photonic crystal cavity and a gain chip, wherein the one-dimensional photonic cavity acts as a resonant mirror that sets corresponding operating wavelength.

3. The refractive index sensor as claimed in any preceding claim, wherein the cladding layer includes a thick silicon oxide cladding to maintain constant corresponding oscillating frequency upon exposure to the input analyte.

4. The refractive index sensor as claimed in any preceding claim wherein the hybrid external cavity laser includes a photonic crystal, micro-ring resonator, distributed bragg reflector or similar photonic integrated circuit component.

5. The refractive index sensor as claimed in claim 1, wherein an additional layer of different thermo-optic co-efficient is deposited on the reflector or waveguide of the reference laser, to enable appropriate matching of the wavelengths of the reference and sensing lasers by varying system temperature, and wherein the additional layer is a polymer layer or a glass coating.

6. The refractive index sensor as claimed in any preceding claim, wherein each of the RI sensitive and reference lasers has oscillating frequencies, such that the difference in respective frequencies is less than the bandwidth of the photodiode.

7. The refractive index sensor as claimed in any preceding claim, wherein the RI sensor is situated within a gas cell.

8. The refractive index sensor as claimed in any preceding claim, wherein the photodiode is integrated either monolithically, using heterogeneous integration, hybrid integration or using flip chip bonding on the chip.

9. The refractive index sensor as claimed in any preceding claim, wherein the RI sensitive laser has the oscillating frequency that varies based on a refractive index change generated by temperature, through the thermo-optic mechanism, so that the RI sensor is used for sensitive temperature measurements.

10. The refractive index sensor as claimed in claim 1, wherein the RI sensitive laser has the oscillating frequency that varies based on at least one of: a refractive index change measured directly in the material of the analyte, or indirectly via the temperature change inducted in the material of the RI sensitive laser, a refractive index change generated by pressure of the fluid interacting with the RI sensitive laser, a refractive index change generated by concentration of the fluid interacting with the RI sensitive laser, or a refractive index change induced by the photothermal effect11. The refractive index sensor as claimed in any preceding claim, wherein the sensing laser and reference laser are narrow linewidth lasers self-injection locked to Photonic integrated circuit based resonator.

12. The refractive index sensor as claimed in any preceding claim further comprising a signal generator for generating a reference signal of a given reference frequency, and an external source to cause further time- varying RI change either directly or indirectly in the input analyte based on the reference frequency, and wherein the photodiode and computing unit performs phase- sensitive detection of the beat signal between the RI sensitive laser and the reference laser by locking onto the known reference frequency, to generate the amplitude and phase of the refractive index change with high sensitivity, in presence of significant noise.

13. The refractive index sensor as claimed in any preceding claim, wherein the evanescent tail of a mode, or portion of the mode, of the RI sensitive laser interacts with the input analyte to cause changes in the oscillating frequency.

14. A method of estimating a refractive index of an input analyte, comprising: providing a reference laser and a RI sensitive laser, wherein each of the reference and RI sensitive lasers is a hybrid external cavity laser formed of a gain element, a waveguide and a reflector, and wherein the reference laser and RI sensitive laser are realised on a single chip, for exposure to identical ambient conditions; covering the waveguide and reflector of the reference laser with a cladding layer to prevent interaction of the reference laser with the input analyte and enable the reference laser to maintain constant oscillating frequency upon exposure to the input analyte; varying an oscillating frequency of the RI sensitive laser based on the refractive index of the input analyte, upon exposure to the input analyte, wherein the reflector of the RI sensitive laser includes an RI sensitive element; generating reference and sensing signals by reference and laser upon exposure of the reference laser to the input analyte, wherein the reference signalhas a constant reference frequency, and the sensing signal has a sensing oscillating frequency that varies based on the refractive index of the input sample; combining the reference and sensing signals by a photodiode, to generate a combined signal that has a beat frequency proportional to the refractive index of the input analyte; and estimating the refractive index of the input sample based on the beat frequency.

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